1. Field of the invention
The invention relates to a method for switching packets, which are transferable by transmission cells and comprise a control code, in a switching system having a number of inputs and a number of outputs, from the input at which the packet appears to a specific output derived from the control code of the packet. The invention also relates to a switching system in which this method can be pre-eminently carried out.
2. Prior art
A method and switching system as indicated above is known from U.S. Pat. No. 4,937,814, referenced under C, discloses a switching system comprising junction switches for transferring data packets from a first bus to a second bus. The junction switch comprises a translation memory for translating packet addresses into new addresses, a comparator for selecting the packets that are to be routed to the second bus, a packet queue memory for storing selected packets until an insertion circuit, after receiving an empty packet code (EPC), places such stored packet in the first arriving empty slot on the other bus by means of a serial/parallel (shift) register.
Another method and switching system as indicated above is disclosed, inter alia, in the publication indicated under C. In this publication a so-called "knockout"--switch is described with which it is possible to route packets, which are transferred via "time slots"--hereinafter termed transmission cells--and which, in addition to their information content, are also provided with a control code, through the switching system from an arbitrary input of the switching system to that output which is indicated by the control code--directly or indirectly--as output destination.
A significant problem in switching systems of the knockout type is the congestion which occurs as soon as packets intended for the same output arrive simultaneously at different inputs. Without an a priori control of the arrival of packets, this problem is unavoidable, while it is precisely the implementation of such a control which makes a switching system appreciably complex. Both the known "knockout" switch and the method and the switching system according to the invention aim to solve this problem using means which are as simple and as effective as possible and which, moreover--because of the large number of inputs and outputs in practice and the desired possibilities for expansion therein--are suitable for a modular architecture. In the method disclosed in the said publication, use is made of so-called "2 2-contention switches" for the concentration of the packets collected. These switches have two inputs and two outputs which are controlled by a "busy bit", which (optionally temporarily) forms part of the control code of the packets. If a packet arrives at the left-hand input, this packet is signalled by reading out the "busy bit" and the left-hand input and the left-hand output are connected to one another, as are also the right-hand input and the right-hand output, and the packet is switched from the left-hand input to the left-hand output, while a packet which may arrive at the right-hand input at the same time is switched to the right-hand output. If an inactive "busy bit" is read out at the left-hand input, which indicates that a packet is not present at that input at that point in time, the right-hand input is connected to the left-hand output, while the left-hand input and the right-hand output are inactive. A packet which may be present at the right-hand input is then thus switched to the left-hand output. In the known switching system a large number of these "contention switches" are assembled to give "concentrators", with which all inputs of the switching system are connected and which each--via a number of output buffers--open into a single output. Packets appearing at the inputs are concentrated, after their control code has been read in "packet filters" by means of the "contention switches" towards a series of internal outputs and then, via the said buffers, to a single (external) output, the output of the switching system derived--directly or indirectly--from their control code.